The present invention relates generally to manufacturing a heat sink for an inverter, and more particularly is a method of manufacturing a porous metal heat sink.
Traction vehicles, such as locomotives or high power off-road vehicles, are driven by electrically powered traction motors which drive the wheels of the vehicle. The traction motors operate on AC power, but the power generated by the engine of the vehicle is DC. The DC potential generated by the engine must therefore be converted from DC to AC current in an inverter. The electric power generation/inversion requires the use of multiple semiconductor devices, and switches to control the semiconductor devices, all of which generate a great deal of heat. To dissipate the heat produced in the inverter, current art vehicles use either water or air cooling systems, or both in combination. These current art methods of cooling lead to several problems.
For any device to be air cooled, there must be adequate space around the device for air to flow in sufficient volume to remove the heat. Since traction motor applications typically utilize three-phase AC current, six IGBT (insulated gate bipolar transistor) switches must be employed. The power requirements of the motors will require that a capacitor bank be present in the inverter, along with the accompanying sensors, etc. The number of components required mandates a significant space requirement which is greatly exaggerated due to the need for space to accommodate air flow around the inverter.
In direct contradiction to the need for open space for cooling air flow is the fact that electrical devices function best in enclosed, non-ventilated environments. This kind of environment reduces the potential of contaminant buildup. Contaminant buildup can not only impede the desired heat transfer, but may also cause an electrical failure of the device. Therefore air cooling directly creates a situation detrimental to the function of the electrical device, in this case an inverter.
Because of the problems caused by air cooling, some current art devices utilize water cooling in the inverter. Water cooling can operate in a more controlled environment, but is generally not readily available. Engines in vehicles of the class which is the subject of this invention, those that utilize electrically powered traction wheels, are usually oil cooled. Thus, utilization of an inverter that requires water cooling leads to the necessity of including a water cooling system in an engine that would not otherwise have it. Still more space is required for the inverter.
Because of the size requirements demanded by the cooling systems of current art inverters, the inverter comprises a large unit contained in a compartment dedicated only to the inverter. This necessitates that lead wires for control and feedback systems must be fairly long, typically anywhere from 2 to 10 feet. Longer wires are by necessity heavier than shorter wires, both in terms of weight and electrical rating. Longer wires significantly increase the potential for distorted signals.
The inventors of this process have discovered a means to utilize an oil-cooled inverter that utilizes the same cooling system as is used for the engine of the vehicle. The improved inverter also drastically reduces the space required for the inverter in the engine compartment.
The improved inverter, which is the subject of a concurrently filed application, said application being incorporated by reference in its entirety herein, utilizes a heat sink which is a significant improvement over the prior art.
The requirements of the heat sink, and the objects of the present invention are as follows:
1. The heat sink must utilize a porous metal media which creates a plurality of pathways uniformly distributed through out the media to allow passage of the heat exchange fluid.
2. The metallic components in the porous metal media must be metallurgically bonded to each other to allow for conduction of heat through the porous media.
3. There must be a large surface-area-to-volume relationship to allow for convection heat transfer from the metallic substrate to the heat exchange fluid.
4. The porous metal media must be able to be metallurgically bonded to a thin molybdenum plate.
5. Once the porous metal media is metallurgically bonded to the molybdenum plate the opposite face must be prepared to allow for metallurgical attachment of the semiconductor devices.
The present invention is a method of manufacturing a heat sink with a porous metallic interior. The porous interior is formed from a large plurality of metal balls bonded together to provide a plurality of fluid flow paths. The metal balls serve as heat sinks to dissipate the heat in the fluid that flows through the heat sink. The metal balls must therefore be in thermally conductive contact with each other, and the bond area between the balls must be as large as is feasible to provide a sufficient heat path for the heat from the fluid.
In the application in which the heat sink of the present invention is used, an inverter for an electric motor, the heat sink housing must be formed from molybdenum. The inverter operates with a large number of silicon based chips. Molybdenum has a thermal expansion rate equal to that of silicon. Therefore, forming the heat sink housing from molybdenum allows the circuitry of the inverter to be mounted directly on the top surface of the heat sink, as the expansion and contraction of the heat sink housing will match that of the silicon substrate of the chips. Since both elements, the heat sink housing and the chip substrate, expand and contract at an equal rate during heating and cooling, the problem of the chip substrate cracking and breaking due to thermal flexing is eliminated.
However, in order to be susceptible to receiving soldered components, as is a key element of the inverter application, the molybdenum surface must be nickel plated. The nickel plating not only allows chips to be soldered to the surface of the heat sink, but also provides an improved bonding surface for the brazing process that secure the heat transfer media. The surfaces to be soldered are external, and an electrodeposited nickel plating is sufficient. However, the porous metallic media is brazed to an internal surface of the housing. To properly nickel plate the interior of the housing, it has been found by the inventors that an electroless process is optimal. When an electroless process is used for the interior of the housing, it is a simple matter to extend the process to the exterior as well.
When the porous metal medium has been placed in the interior of the heat sink housing, the end caps (manifolds) are bonded to the main heat sink body. This joint between the main body of the housing and the manifolds has minimal mechanical performance requirements, but the joint does have to seal the unit with a sufficiently strong bond so that pressurized fluid can flow through the heat sink.
For the preferred embodiment of the present invention, the porous metal medium is constructed by packing relatively small (approximately {fraction (3/32)}xe2x80x3 diameter) balls together in the heat sink housing. The balls must be bonded together and the bond contact area must be sufficiently large to allow for heat conduction from ball to ball. The metallurgical bonding between the balls is achieved by plating a brazing compound of sufficient volume so that during a brazing cycle, a sufficient volume of liquid is produced to allow wetting at contact points between the balls to increase the size of the conductive heat transfer paths.
In the preferred embodiment, a copper ball is used as the basis for the porous metal medium in order to optimize conductive heat transfer, and because copper has the necessary ductility to allow the manufacture, with currently available technology, of balls of the small size required. However, copper alloys are susceptible to hydrogen embrittlement, so the composition of the balls must be controlled to allow for the necessary thermal processing required during brazing. An OFHC (Oxygen Free High Conductivity) copper alloy was selected. A copper-silver eutectic brazing compound is formed on the surface of the balls during the thermal processing.
An advantage of the present invention is that a heat sink that utilizes a porous metal medium as the heat exchange element is formed. Fluid used to cool an engine of a vehicle flows through the heat sink to serve as a coolant for an inverter.
Another advantage of the present invention is that the heat sink accomplishes the same amount of heat transfer as prior art devices with vastly larger size and weight requirements.
A still further advantage of the present invention is that it provides a molybdenum based housing to which electronic components can be soldered.
These and other objects and advantages of the present invention will become apparent to those skilled in the art in view of the description of the best presently known mode of carrying out the invention as described herein and as illustrated in the drawings.